1. Field of the Invention
The present invention relates to a method for producing an electrode for alkaline batteries, which is formed by coating and adhering a slurry of an active material comprising a powder of active material, a binder, and a solvent for the binder on an electrically conductive core body.
2. Description of the Related Art
Recently, with the increase in compact mobile appliances, demand for chargeable and dischargeable secondary cells (batteries) is increasing. In particular, the demand for high capacity low-cost alkaline batteries such as nickel-hydrogen batteries is rapidly increasing with the appliances decreasing in size and thickness, and which are increasing space efficiency. The electrodes for use in this type of alkaline batteries are obtained by coating and adhering a slurry of active material, which is formed by kneading a powder of active material with a water-soluble binder and a solvent for the water-soluble binder, i.e., pure water or water, on the both sides of an electrically conductive core body made from a punching metal and the like retaining an active material layer. In general, after the active material slurry is coated and adhered to the electrically conductive core body, the resulting body is subjected to a step of naturally drying at an ordinary temperature (about 20xc2x0 C.).
However, on naturally drying the electrode for alkaline batteries having coated and adhered thereon the slurry of active material, it had been found that the drying rate is so low, and that it generally takes a long duration of time of about 5 to 6 hours to dry the electrode for alkaline batteries. This led to a problem of poor production efficiency of batteries for alkaline batteries.
In order to overcome the above problem, there has been proposed a method comprising coating and adhering a slurry of active material on the both sides of an electrically conductive core body, and drying the resulting body at a high temperature (about 60xc2x0 C.). By drying the electrode for alkaline batteries having the slurry of active material coated and adhered thereon at high temperature in this manner, it was found that the dry time is reduced to 15 to 30 minutes. This resulted in an increase in production efficiency of electrodes for alkaline batteries.
In order to increase the production efficiency by speeding up the dry rate of the electrode for alkaline batteries, the electrode for alkaline batteries must be dried at a high temperature as described above. When the electrode for alkaline batteries is dried at a high temperature, the evaporation rate of water increases as such that the water contained in the active material layer moves rapidly from the interior of the electrode for alkaline batteries to the surface (the side of the drying plane). However, if water contained in the active material layer is rapidly moved, the binder contained in the active material layer also moves with the movement of water. As a result, the binder thus moved solidifies segregated on the surface of the electrode for alkaline batteries. Hence, there occurred a phenomenon that the amount of binder of the active layer in the vicinity of the electrically conductive core body located at the center of the electrode for alkaline batteries becomes too scarce.
In case the amount of binder contained in the active layer in the vicinity of the electrically conductive core body becomes too small, the adhesion power among the active materials present in the vicinity of the electrically conductive core body or the adhesive power between the electrically conductive core body and the active material decreases, and this resulted in a problem that the active material layer easily drops off from the electrically conductive core body.
In the light of such circumstances, the present invention has been made to overcome the above problems. More specifically, an object of the present invention is to provide a production method for the electrode of alkaline batteries, in which the temperature for drying the electrode for alkaline batteries having coated and adhered thereon the active material is elevated to increase the production efficiency, but which is yet capable of suppressing the drop in strength of the electrode for alkaline batteries and of providing a high quality alkaline battery.
In order to achieve the object above, the present invention provides a production method for an elec. formed by coating and adhering a slurry of an active material comprising a powder of active material, a binder, and a solvent for the binder on an electrically conductive core body, said method comprising: a step of preparing a slurry of an active material by mixing the powder of active material with a binder and a solvent for the binder; a step of heating the slurry of the active material; and a step of coating and adhering slurry, in which the heated slurry is coated and adhered to the electrically conductive core body.
By coating and adhering the warm active material slurry to the electrically conductive core body after heating the active material slurry in the manner above, the temperature gradient between the temperature of the atmosphere inside the drying furnace and that of the active material slurry can be reduced even if the active material slurry is fed inside the drying furnace for heating and drying. Thus, since the movement of the binder inside the active material slurry above is suppressed, the decrease in the amount of the binder can be thereby prevented from occurring. As a result, the adhesive force among the powder particles of the active material in the vicinity of the electrically conductive core body as well as the adhesive force between the electrically conductive core body and the powder of active material can be maintained. Thus, the adhesive strength of the elec. can be improved as to prevent the falling off of the active material layer from occurring from the electrically conductive core body.
Furthermore, in order to achieve the effect of heating the active material slurry according to the invention, it is particularly effective in case the process is applied to a method for producing an electrode for alkaline batteries comprising a drying step for drying the electrode comprising an electrically conductive core body having coated and adhered thereon the heated active material slurry, and a pressing step for rolling the dried electrode. The temperature for heating the slurry in the active material slurry heating step must be set lower than the drying temperature.
In this case, however, if the temperature of the heated active material slurry is lower than 30xc2x0 C., the temperature gradient between the temperature of the atmosphere inside the drying furnace and that of the active material slurry does not decrease in the later step of drying. Hence, inside the drying furnace, the binder inside the active material slurry moves to the surface of the electrode as to decrease the amount of binder inside the active material layer in the vicinity of the electrically conductive core body. This results in a drop in adhesion strength between the electrically conductive core body and the powder of active material. Thus, the temperature of heating the slurry in the heating step for the active material slurry should be set to 30xc2x0 C. or higher. However, if the slurry is heated to the decomposition temperature of the binder or higher, the binder can no longer exhibit the function of the binder. Hence, the upper limit of the temperature for heating the active material slurry should be set at a temperature as such that the binder would not undergo decomposition.